Recently, in a process of manufacturing a semiconductor device, a substrate to be processed (or a target substrate) such as a silicon wafer or a compound semiconductor wafer has been increased in diameter and reduced in thickness. However, a target substrate having a large diameter and a small thickness may be bent or cracked when transferred or polished. Due to this, a support substrate such as a glass substrate is bonded to a target substrate to reinforce the target substrate.
For example, there is a method of holding a target substrate and a glass substrate respectively with an upper chuck and a lower chuck, placing the upper chuck and the lower chuck to be close to each other, and pressing the target substrate and the glass substrate to bond both substrates. A glue (or an adhesive), for example, is applied to the surfaces of the target substrate and the glass substrate, and as mentioned above, the target substrate and the glass substrate are bonded through pressing.
There has been also suggested a method of installing a heating mechanism in the upper chuck and the lower chuck and heating the target substrate and the glass substrate to bond them.
However, in the aforementioned methods, when an electrostatic chuck is employed as a holding unit of the glass substrate, the glass substrate is electrostatically held by the electrostatic chuck at a high temperature. Therefore, sodium may be extracted from the glass substrate, which may result in defects, for example, degradation of a surface of the electrostatic chuck, corruption of the glass substrate, or the like.